The Anatomy of the Human Peritoneum and Abdominal Cavity / Considered from the Standpoint of Development and Comparative Anatomy

by George S. Huntington

[Fig. 440] shows the structures in Hapale jacchus, one of the marmosets, and [Fig. 443] illustrates the typical cæcum of this form in Ateles ater, the black-handed spider monkey.

2. The cæcum and appendix of man and of the anthropoid apes can be regarded as a reduction form of this type ([Fig. 465], III, A, 2). Arrest of development of the terminal portion converts the distal segment of the cæcal pouch into an appendix whose relation to the apex of the funnel-shaped proximal segment or cæcum proper is seen in its pure form in the human embryo ([Figs. 512] and [525]). With the further development of the cæcum the sharper demarcation between it and the appendix results ([Figs. 517] and [518]). The displacement of the root of the appendix cephalad and to the left, toward the lower margin of the ileo-colic junction, as it is usually seen in adults, is due to the relatively greater growth of the right terminal sacculation of the cæcum compared with the left (cf. types of cæca, [p. 248]). Throughout these changes the initial crescentic curve of the cæcum, turning its concavity upwards and to the left, can be recognized by tracing the course of the longitudinal colic muscular bands. The cæca and appendices of the anthropoid apes present the same characters. The structures in the orang, chimpanzee, gorilla and gibbon are shown in [Figs. 455]-[464].

B. The Æluroid and Arctoid groups of the Carnivora and the Pinnipedia constitute a very complete and instructive series illustrating the gradual reduction of the cæcum from the capacious pouch of the primitive type and its final complete elimination from the organism ([Fig. 465], III, B).

In Hyæna ([Fig. 416]), the large cæcum with undiminished caliber of the terminal portion persists in its full development, as seen in the Marsupials furnishing the fundamental type ([Fig. 465], III). The same type of cæcum is found in the lion ([Fig. 417]), the only true cat in which the cæcal apparatus has not undergone extensive reduction. Phylogenetically the presence of a capacious and uniform cæcal pouch in these two animals is exceedingly important and indicates that this type of cæcum represents the ancestral form common to the æluroid carnivore group, which, in the remaining living representatives, has become reduced in response to the influence which the character of the food has on the structure of this portion of the intestinal canal. The two instances of persistence of the primal type are all the more important as exceptions to the rule which is otherwise universal throughout the group.

1. The first example of this reduction ([Fig. 465], III, B, 1) is encountered in the Aard-Wolf, Proteles lalandii, a near relative of hyæna ([Fig. 406]). The cæcum in this animal is considerably shortened, although still of fairly large and uniform caliber.

A similar type of cæcal reduction is encountered in the Pinnipede Carnivora. [Fig. 396] shows the ileo-colic junction and the short blunt cæcum of the harbor seal, Phoca vitulina.

2. The cæcum of the typical Felidæ, other than the lion, is short and the terminal portion much reduced in caliber, constituting in many forms a species of pointed rudimentary appendix ([Fig. 465], III, B, 2). [Fig. 401] represents the typical feline cæcum as seen in the puma, Felis concolor. Among the smaller Æluroid Carnivora related to the true cats, as the civets and ichneumons, the terminal reduction of the short cæcum is still more marked, as seen for example in Herpestes griseus ([Figs. 404] and [405]).

3. In the Arctoid group of Carnivora ([Fig. 465], III, B, 3 and 4) the reduction of the cæcal apparatus has been carried to the complete elimination of the pouch, restoring the primitive type of a straight intestinal tube without diverticulum as encountered above in some of the Edentates ([Figs. 356] and [357]).

In some forms allied to the true bears, such as Procyon, Bassaris, Cercoleptes, Taxidea and Nasua, the ileo-colic junction is marked externally by a slight constriction and internally by the projection of an annular pylorus-like valve ([Figs. 407]-[409]). The transition from the thin-walled ileum to the thick muscular walls of the large intestine is abrupt. The latter is very short and usually increases in caliber as it approaches the anal orifice. The mucosa of the terminal ileum presents very commonly one or two large oval areas of agminated follicles near the ileo-colic junction. The mucous membrane of the large intestine is thrown into prominent longitudinal folds. [Fig. 408] shows the intestine of the brown coati, Nasua rufa, opened on each side of the ileo-colic transition.